Download Ruminant Digestive System

Copyright 1997 by Drew A. Vermeire, Ph.D., PAS All Rights Reserved.
Nutrition Series Number 1
Single Issue Price $4.00
UNDERSTANDING THE RUMINANT DIGESTIVE SYSTEM
Fermentation System on the Hoof
The dynamic and amazing ruminant digestive system provides cattle and other ruminants with some unique
advantages. It also provides livestock producers with opportunities for feeding economies. Ruminants can utilize
feedstuffs which are otherwise unusable by man and would create disposal problems, were it not for cattle. The
unique feature of all true ruminants is a large pre-gastric fermentation in the rumen and reticulum which allows the
ruminant to utilize fibrous feeds. Understanding how the ruminant digestive system works helps us develop
management and nutritional programs to maximize performance of beef cattle with minimum cost of production.
What is Digestion?
Digestion is the breakdown of feed to its component nutrients, making them small or soluble enough to be
absorbed into the bloodstream. Digestion includes mechanical forces such as chewing and muscle contractions that
physically break feed into particles, and chemical action such as stomach acid and attack by enzymes that are
produced either by the digestive tract or by microorganisms.
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1 Mouth
2 Esophagus
3 Rumen
4 Reticulum
5 Omasum
6 Abomasum
7 Small Intestine
8 Cecum
9 Large Intestine/Colon
10 Rectum
11 Heart
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The first step in the process of digestion involves the mouth where tremendous amounts of saliva are
secreted daily by cattle, depending on quantity and type of diet. Saliva contains water, mucin, protein, salts and
bicarbonate to lubricate and begin the breakdown of ingested feed particles. Bicarbonate helps maintain rumen pH
in the range of 5.8 - 7.0. Cattle fed diets which are high in roughage produce up to 200 quarts of saliva per day,
while cattle fed high concentrate diets may produce only 30-80 quarts.
Do Cattle Have Four Stomachs?
Contrary to popular belief, cattle don’t really have four stomachs. They have a series of four compartments:
Rumen, Reticulum, Omasum and Abomasum. Each compartment has specific functions. The rumen and
reticulum are the site of fermentation, while the omasum is thought to masticate feed and absorb water. The
abomasum, or “true stomach”, initiates protein digestion using hydrochloric acid and enzymes.
From the code of the old west:
Always take a good look at what you’re about to eat.
It’s not so important to know what it is, but it’s critical to know what it was.
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Rumen Fermentation & Rumination
First, no enzymes or other digestive secretions
are produced by the rumen. The warmth, moisture,
slightly acidic pH (5.8-7.0) and lack of oxygen inside the
rumen are ideal for the growth of bacteria and protozoa,
commonly called the "rumen microbes".
Feed is
fermented by these microbes. The animal uses many of
the fermentation by-products, such as volatile fatty acids
(VFAs), as sources of nutrients. Other fermentation byproducts, such as carbon dioxide and methane, are lost
when the cattle eructate (belch).
Ingesta can flow freely between the two
compartments of the rumen and reticulum which have a
combined capacity of about 33 gallons in a 1250 lb cow.
Heavy, dense particles, such as whole corn, sink to the
bottom of the rumen. Light, fluffy feeds, such as hay,
float on the top of the rumen forming a “mat” or “raft”.
Particles which have specific gravity close to 1.0 leave
the rumen with the fluid which passes into the omasum.
Some rumen microbes attach to particles in the rumen,
while others are free in the fluid. There are hundreds of
species of microorganisms in the rumen and the species
which predominate are dependant on the diet being fed.
Starch-digesting microbes predominate when a highconcentrate diet is fed, while cellulytic bacteria
predominate when a roughage-based diet is fed. A
change in diet disrupts the microbes and it takes 2 - 3
weeks for the microbes to adapt to the new diet.
The ruminant regurgitates and rechews feed for
greater physical/mechanical breakdown. This process is
called “rumination” or “chewing the cud”. Additional
chewing provides more surface area for the microbes to
attack, the next time it reaches the rumen.
Results of Rumen Fermentation:
• Digestion of fiber and starch.
• Production, absorption and utilization of volatile fatty
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acids (VFA's) to satisfy approximately 70% of the
animal's energy needs.
Amino acid degration and nonprotein nitrogen
utilization.
Microbial synthesis of water soluble vitamins.
Production and loss of rumen gases, primarily carbon
dioxide and methane. Large volumes of these gases
are produced by microbial metabolism and lost by
belching.
Omasum - 5 Gallon Capacity
From the rumen-reticulum, rumen contents flow
to the omasum which is thought to aid in reducing the
size of food particles that were not completely fermented,
and water is absorbed.
Abomasum - 4 Gallon Capacity
The fourth compartment is often referred to as
the "true stomach" since its function is similar to nonruminant animals such as humans, swine, dogs and cats.
From this point on, digestion is similar for ruminants and
non-ruminants. The abomasum secretes hydrochloric
acid (pH 1-3) and pepsin - an enzyme which breaks
Small Intestine - 17 Gallon Capacity
Digesta is now referred to as “chyme”. Bile
salts, secreted by the liver, raise pH and emulsify fats in
the small intestine.
Many enzymes break down
proteins, fats, and carbohydrates for absorption.
Vitamins and minerals are absorbed here as well.
Rumen microbes are digested and absorbed in the
abomasum and small intestine along with other
feedstuffs.
Large Intestine - 9.2 Gallon Capacity
The large intestine includes the cecum, colon
and rectum. An active bacterial fermentation occurs in
both the cecum and colon and produces VFAs which are
then absorbed. A major function of the colon is
reabsorption of water. Remaining undigested feed,
microbial cells, secretions and abraded tissue are
passed through the rectum and excreted.
Ruminants vs Non-ruminants
Digestion in the ruminant animal is similar to
that of most nonruminants, with the exception that feeds
are subjected to a pre-gastric fermentation by bacteria
and protozoa in the rumen and reticulum. Pre-gastric
fermentation gives the ruminant animal the capability of
using feedstuffs which nonruminants cannot use, such
as forages and nonprotein nitrogen. The ability of the
ruminant to utilize forages provides man the opportunity
to harvest meat, milk, and fiber from marginal land
areas. Energy-dense feeds, such as corn, can be
utilized by ruminants when it is economically feasible to
feed grains to ruminants. Human food by-products such
as food processing wastes, milling by-products, distillery
by-products, oil seed by-products, brewing by-products
and bakery by-products can be fermented and absorbed
as nutrient sources by ruminants.
Pre-ruminants and Ruminants
At birth, ruminants have a digestive system
resembling the non-ruminant species. As the neonatal
calf nurses the cow, a specialized structure called the
“esophageal groove” closes and bypasses the rumen as
it directs milk from the esophagus directly into the
omasum and abomasum. The calf uses milk more
efficiently by digesting it in the abomasm and small
intestine. Once the calf begins consuming dry feed,
however, bacteria fermentation causes the rumen to
grow and develop. It grows in size and the lining
develops projections called “papillae” which greatly
increase absorptive area.
Getting
More
Fermentation
From
the
Rumen
Like other fermentation systems, the rumen
operates at peak efficiency when frequent, small doses
of feed are provided. Big doses can cause acidosis and/
or bloat. Consistency is important to maintaining proper
pH and maintaining a healthy rumen. Changes in feed
composition should be gradual - preferably 2-3 weeks.
Feed should be provided at the same time each day and
the amount should be changed in small increments over